Reverse tone STI formation

ABSTRACT

A method includes forming a hard mask over a substrate, patterning the hard mask to form a first plurality of trenches, and filling a dielectric material into the first plurality of trenches to form a plurality of dielectric regions. The hard mask is removed from between the plurality of dielectric regions, wherein a second plurality of trenches is left by the removed hard mask. An epitaxy step is performed to grow a semiconductor material in the second plurality of trenches.

This application is a divisional of U.S. patent application Ser. No.13/298,112, entitled “Methods for Epitaxially Growing Active Regionsbetween STI Regions,” filed on Nov. 16, 2011, which application isincorporated herein by reference.

BACKGROUND

In the formation of integrated circuits, Shallow Trench Isolation (STI)regions are used in semiconductor wafers to define active regions.Integrated circuit devices such as transistors may then be formed at thesurfaces of the active regions.

In the existing STI formation processes, the STI regions are formed byforming trenches in a silicon substrate first. The formation of thetrenches includes forming a pad oxide layer over the silicon substrate,and forming a silicon nitride layer over the pad oxide layer. Thesilicon nitride layer, the pad oxide layer, and the silicon substrateare then etched to form the trenches. The trenches are filled with adielectric material. A Chemical Mechanical Polish (CMP) is thenperformed to remove excess dielectric material that is over the siliconnitride layer. The portions of the dielectric material left in thesilicon substrate thus form STI regions. The portions of the siliconsubstrate between the STI regions are the active regions. The remainingsilicon nitride layer and the pad oxide layer are then removed. It hasbeen found that in certain processes, such as in double-patterningprocesses, the thicknesses of the STI regions are not uniform. Large STIregions and small STI regions may have a significant difference inthicknesses.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the embodiments, and the advantagesthereof, reference is now made to the following descriptions taken inconjunction with the accompanying drawings, in which:

FIGS. 1 through 13 are cross-sectional views of intermediate stages inthe manufacturing of Shallow Trench Isolation (STI) regions and activeregions in accordance with various embodiments.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the embodiments of the disclosure are discussedin detail below. It should be appreciated, however, that the embodimentsprovide many applicable inventive concepts that can be embodied in awide variety of specific contexts. The specific embodiments discussedare merely illustrative, and do not limit the scope of the disclosure.

Methods for forming isolation regions in semiconductor substrates andactive regions between isolation regions are provided. The intermediatestages of manufacturing the isolation regions and the active regions areillustrated in accordance with embodiments. Variations of theembodiments are then discussed. Throughout the various views andillustrative embodiments, like reference numbers are used to designatelike elements.

FIGS. 1 through 13 illustrate cross-sectional views of intermediatestages in the formation of active regions and isolation regions inaccordance with various embodiments. FIG. 1 illustrates a structureincluding substrate 20 and overlying layers. Substrate 20 may be formedof a semiconductor material such as silicon, silicon germanium, or thelike, and may be a bulk substrate or a semiconductor-on-insulator (SOI)substrate. In some embodiments, substrate 20 is a crystallinesemiconductor substrate such as a crystalline silicon substrate. Padoxide layer 22 and hard mask 24 are formed over substrate 20. Inaccordance with embodiments, pad oxide layer 22 comprises silicon oxide.Hard mask 24 may be formed of silicon nitride, polysilicon, or the like.The thickness of pad oxide layer 22 may be between about 50 Å and 150 Å.The thickness of hard mask 24 may be greater than about 0.07 μm. In someembodiments, the combined thickness of hard mask 24 and pad oxide layer22 may be selected to be substantially equal to, or greater than, thedesirable thickness T2 of isolation regions (STI regions) 65 in FIG. 12.

Hard mask 32 is formed over substrate hard mask 24. Hard mask 32 maycomprise an Ashing Removable Dielectric (ARD) material, and hence isreferred to as ARD 32 hereinafter, although it may also be formed ofother materials. In some embodiments, ARD 32 includes amorphous carbon.Plasma enhanced (PE) oxide 34, which may be a silicon oxide formed usingPlasma Enhanced Chemical Vapor Deposition (PECVD), is formed over, andmay adjoin, ARD 32. In some embodiments, silicon oxynitride layer 36 isformed over PE oxide 34. PE oxide 34 and silicon oxynitride layer 36 maybe used for lithographic purposes, for example, for reducing thereflection of the yellow light used in the exposure of the overlyingphoto resist. It is appreciated that layer 34 and/or layer 36 may alsobe formed of other materials.

ARD 38, silicon oxynitride layer 40, and bottom anti-reflective coating(BARC) 42 may be formed over silicon oxynitride layer 36. ARD 38 may beformed of the same material as ARD 32. Throughout description, ARD 38 isalternatively referred to as a mandrel layer since it is used forforming mandrels 46 (not shown in FIG. 1, please refer to FIG. 2).Layers 38, 40, and 42 may be replaced by other materials, and the numberof layers may also be different from what is shown in FIG. 1.

FIGS. 1 and 2 also illustrate a first lithography process for patterningARD 38. Photo resist 44 is formed over BARC 42, and is then patterned.Layers 38, 40, 42, and 44 are used to form patterns with small pitches,which may be less than the minimum pitch allowed by the lithographyprocess used for forming the integrated circuits. Layers 32, 34, and 36are used to transfer the small pitches to substrate 20. In someembodiments, the minimum pitch P1 of photo resist 44 may be close to, orequal to, the minimum pitch allowed by the technology for forming photoresist 44 and for performing the etch using photo resist 44 as anetching mask.

As illustrated in FIG. 2, BARC 42, silicon oxynitride layer 40, and ARD38 are etched, for example, using plasma-assisted dry etching, followedby the removal of photo resist 44 and BARC 42. The remaining portions of38 are referred to as mandrels 46 hereinafter. In the resultingstructure, leftover portions of silicon oxynitride layer 40 may remainon top of mandrels 46. The minimum pitch of mandrels 46 may besubstantially equal to minimum pitch P1 of photo resist 44 (FIG. 1).

Next, as shown in FIG. 3, spacer layer 50 is deposited using a conformaldeposition method. In some embodiments, spacer layer 50 is depositedusing Atomic Layer Deposition (ALD), which may form a high quality filmthat has a low etching rate. The ALD may be performed usingDiChloroSilane (DCS) and ammonia as precursors, and the resulting spacerlayer 50 may include silicon nitride or silicon-rich nitride. Inalternative embodiments, other conformal deposition methods, such asLow-Pressure Chemical Vapor Deposition (LPCVD), may be performed.Thickness T1 of spacer layer 50 may be less than a half of, and may beclose to about a third of, pitch P1 of mandrels 46.

FIGS. 4 and 5 illustrate a second lithography process for patterningspacer layer 50. Referring to FIG. 4, bottom layer 54 is formed overspacer layer 50. Bottom layer 54 may contain a polar component such as apolymer with hydroxyl or phenol groups. In an embodiment, bottom layer54 comprises an i-line photo resist. Alternatively, bottom layer 54comprises a deep Ultra-Violet (UV) photo resist including polymershaving hydroxystyrene groups. Middle layer 56 is then formed over bottomlayer 54. Middle layer 56 may be formed of an oxide-like photo resist.Bottom layer 54 and middle layer 56 may be formed using spin-on coating.Followed by the formation of middle layer 56, photo resist 58 is formedand patterned.

Middle layer 56 and bottom layer 54 are patterned according to thepattern of photo resist 58, and hence the structure in FIG. 5 is formed.In an exemplary process for forming the structure in FIG. 5, portions ofmiddle layer 56 and bottom layer 54 that are not covered by photo resist58 are etched first, until top portions 50A (please refer to FIG. 4) ofspacer layer 50 are exposed. Top portions 50A are located over andaligned to mandrels 46. At this time, portions 54A (FIG. 4) of bottomlayer 54 still remain. Next, top portions 50A and silicon oxynitridelayer 40 are etched, until mandrels 46 are exposed. The remainingportions 54A of bottom layer 54 and mandrels 46 are then removed, forexample, using plasma-assisted ashing. Photo resist 58 and the remainingportions of middle layer 56 and bottom layer 54 are then removed. Theresulting structure is shown in FIG. 5. It is appreciated that theabove-discussed process for patterning spacer layer 50 is merely anexemplary process, and alternative processes may be used.

In FIG. 5, the remaining portions of spacer layer 50 include somesidewall portions that are on the opposite sidewalls of the mandrels 46as in FIG. 4. Optionally, some top portions 50A of spacer layer 50 mayremain. Throughout the description, the sidewall portions of spacerlayer 50 are alternatively referred to as sidewall spacers 60. Pitch P2sidewall spacers 60 may be as small as a half of pitch P1 of mandrels 46in FIG. 2.

FIGS. 6 and 7 illustrate a third lithography process for furtherpatterning spacer layer 50. In FIG. 6, bottom layer 64 and middle layer66 are formed, followed by the formation of photo resist 68. Bottomlayer 64 may be formed of a material selected from the same group ofmaterials for forming bottom layer 54. Middle layer 66 may also beformed of a material selected from the same group of materials forforming middle layer 56. In some embodiments, bottom layer 64 and middlelayer 66 are formed of the same materials as bottom layer 54 and middlelayer 56, respectively.

Next, as shown in FIG. 7, photo resist 68 is used as an etching mask toremove some of sidewall spacers 60, while some other sidewall spacers 60remain not removed. Bottom layer 64, middle layer 66, and photo resist68 are then removed.

In subsequent steps, sidewall spacers 60 and the remaining portions ofspacer layer 50 are used as an etching mask to perform patterning.During the patterning, the underlying silicon oxynitride layer 36, PEoxide layer 34, ARD layer 32, hard mask 24, and pad oxide 22 arepatterned. Accordingly, the pattern of sidewall spacers 60 and theremaining portions of spacer layer 50 is transferred into hard mask 24and pad oxide 22. The remaining portions of silicon oxynitride layer 36,PE oxide layer 34, and ARD layer 32 are then removed. FIG. 8 illustratesthe resulting structure. In some embodiments, portions of top surface20A of substrate 20 may be exposed through the remaining hard mask 24and pad oxide 22. In alternative embodiments, the exposed portions ofpad oxide layer 22 may be left un-etched, as illustrated by dashedlines, which represent the top surfaces of the remaining pad oxide layer22.

Referring to FIG. 9, dielectric material 65 is filled into the spacesbetween hard mask portions 24 and pad oxide 22. The top surface ofdielectric material 65 may be higher than the top surface of hard maskportions 24. In accordance with some embodiments, dielectric material 65is filled by spin-on coating. A curing process, such as thermal curingprocess, is then performed to cure dielectric material 65. In someembodiments, dielectric material 65 comprises silicon oxide.

FIG. 10 illustrates a planarization step. In an embodiment, a ChemicalMechanical Polish (CMP) is performed to remove excess portions ofdielectric material 65, so that the top surfaces of the remainingdielectric material 65 are level with the top surfaces of hard maskportions 24. An anneal step may then be performed on the structure inFIG. 10. In an exemplary anneal process, the annealing temperature isbetween about 650° C. and about 1,100° C., and the annealing duration isbetween about 30 minutes and about 120 minutes. The resulting dielectricmaterial 65 is alternatively referred to as isolation regions 65 or STIregions 65 hereinafter.

FIG. 11 illustrates the removal of remaining hard mask portions 24 andthe underlying portions of pad oxide layer 22. Accordingly, trenches 67are formed between STI regions 65. Top surface 20A of semiconductorsubstrate 20 are exposed through STI regions 65.

Referring to FIG. 12, an epitaxy is performed to grow epitaxy regions 69in trenches 67, wherein the epitaxy is started from substrate 20. Insome embodiments, the epitaxy is selective, and no epitaxy regions aregrown from STI regions 65. Epitaxy regions 69 may comprise crystallinesilicon, crystalline silicon germanium, III-V compound semiconductormaterials, silicon carbon, or the like. Epitaxy regions 69 may includeessentially the same material as underlying substrate 20. For example,when substrate 20 is a crystalline silicon substrate, epitaxy regions 69may also be crystalline silicon regions. It is noted that even ifepitaxy regions 69 and substrate 20 are formed of a same material,noticeable interfaces 69B may be formed between epitaxy regions 69 andsubstrate 20. In the resulting structure, epitaxy regions 69 act as theactive regions, while STI regions 65 define the boundaries of activeregions 69. The top surface of epitaxy regions 69 may be substantiallylevel with, or slightly lower than, the top surfaces of STI regions 65.Alternatively, the top surfaces of epitaxy regions 69 may be lower thanthe top surfaces of STI regions 65.

The structure shown in FIG. 12 may then be used to form active devices.For example, planar transistors, Fin Field-Effect Transistors (FinFETs),diodes, or the like, may be formed on active regions 69. FIG. 13illustrates an exemplary planar transistor 70. It is realized that thestructures in accordance with embodiments may be used to form FinFETs.For example, an etch step may be performed to recess the top surfaces ofSTI regions 65, until the top surfaces of STI regions 65 are lower thanthe top surfaces of active regions 69. The portions of active regions 69over the top surfaces of STI regions 65 are the fins, on which theFinFETs may be formed.

In the embodiments, the STI regions are not formed by etching asubstrate to form trenches, and filling the trenches to form STIregions. Instead, a reversed-tone method is used, wherein the patternsof active regions are defined first by forming STI regions, and then anepitaxy is performed to grow the active regions. Experiment resultsindicated that by using the methods in accordance with embodiments, thelarge-area STI regions and small-area STI regions on a same chip or asame wafer have more uniform thicknesses. In addition, several processsteps in the existing process may be omitted. For example, the In-SituSteam Generation (ISSG) step, which was used to eliminate the surfacesilicon layer that is adversely affected by the pad oxide removal andhard mask removal process, may be omitted. The active regions formed inaccordance with embodiments have a high quality.

In accordance with embodiments, a method includes forming a hard maskover a substrate, patterning the hard mask to form a first plurality oftrenches, and filling a dielectric material into the first plurality oftrenches to form a plurality of dielectric regions. The hard mask isremoved from between the plurality of dielectric regions, wherein asecond plurality of trenches is left by the removed hard mask. Anepitaxy step is performed to grow a semiconductor material in the secondplurality of trenches.

In accordance with other embodiments, a method includes forming a padoxide layer over a semiconductor substrate, forming a hard mask over thepad oxide layer, forming a mandrel layer over the hard mask, performinga first lithography process to pattern the mandrel layer and to form aplurality of mandrels, and forming a spacer layer. The spacer layercomprises top portions over the mandrels, and sidewall portions onsidewalls of the mandrels. The spacer layer is patterned to leave thesidewall portions of the spacer layer. The hard mask and the pad oxidelayer are etched to form hard mask patterns and pad oxide patterns,wherein the step of etching is performed using the sidewall portions ofthe spacer layer as an etching mask. The sidewall portions of the spacerlayer are then removed. The spaces between the hard mask patterns andthe pad oxide patterns are filled with a dielectric material. The hardmask patterns and the pad oxide patterns are removed. An epitaxy step isperformed to grow a semiconductor material in the spaces left by theremoved hard mask patterns and the pad oxide patterns.

In accordance with yet other embodiments, a method includes formingdielectric patterns on a top surface of a semiconductor substrate,wherein portions of the semiconductor substrate are exposed throughspaces between the dielectric patterns. An epitaxy is performed to growepitaxy regions in the spaces, wherein the epitaxy regions are grownfrom the semiconductor substrate. The epitaxy regions and thesemiconductor substrate are formed of essentially a same semiconductormaterial.

Although the embodiments and their advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the embodiments as defined by the appended claims. Moreover,the scope of the present application is not intended to be limited tothe particular embodiments of the process, machine, manufacture, andcomposition of matter, means, methods and steps described in thespecification. As one of ordinary skill in the art will readilyappreciate from the disclosure, processes, machines, manufacture,compositions of matter, means, methods, or steps, presently existing orlater to be developed, that perform substantially the same function orachieve substantially the same result as the corresponding embodimentsdescribed herein may be utilized according to the disclosure.Accordingly, the appended claims are intended to include within theirscope such processes, machines, manufacture, compositions of matter,means, methods, or steps. In addition, each claim constitutes a separateembodiment, and the combination of various claims and embodiments arewithin the scope of the disclosure.

What is claimed is:
 1. A method comprising: forming a pad oxide layerover a semiconductor substrate; forming a hard mask over the pad oxidelayer; forming a mandrel layer over the hard mask; performing a firstlithography process to pattern the mandrel layer and to form a pluralityof mandrels; forming a spacer layer, wherein the spacer layer comprisestop portions over the mandrels, and sidewall portions on sidewalls ofthe mandrels; patterning the spacer layer to leave the sidewall portionsof the spacer layer; etching the hard mask and the pad oxide layer toform hard mask patterns and pad oxide patterns, wherein the step ofetching is performed using the sidewall portions of the spacer layer asan etching mask; removing the sidewall portions of the spacer layer;filling spaces between the hard mask patterns and the pad oxide patternswith a dielectric material; removing the hard mask patterns and the padoxide patterns; and performing an epitaxy to grow a semiconductormaterial in spaces left by the removed hard mask patterns and the padoxide patterns.
 2. The method of claim 1, wherein the step of patterningthe spacer layer comprises two lithography processes.
 3. The method ofclaim 1, wherein the step of filling the spaces comprises a spin-oncoating step, and a curing step after the spin-on coating step to curethe dielectric material.
 4. The method of claim 1, wherein after thestep of epitaxy, a top surface of the semiconductor material issubstantially level with a top surface of the dielectric material. 5.The method of claim 1 further comprising; after the step of filling thespaces with the dielectric material, performing a planarization to levela top surface of the dielectric material and top surfaces of the hardmask patterns; and after the planarization, performing an annealing toanneal the dielectric material.
 6. The method of claim 1, wherein thehard mask comprises polysilicon.
 7. The method of claim 1, wherein thehard mask comprises silicon nitride.